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* Some glycogen is stored in the liver where it flows through the blood to all tissues. The human liver typically stores between 90 and 160 grams of Glycogen, or 350 to 650 Calories.
* Blood typically contains less than 20 calories of glucose. (This assumes 5 liters of blood and 100mg/dL of blood glucose, which is 5g of glucose.)
* Glycogen can also be created from protein [[Protein]] via a process called [http://en.wikipedia.org/wiki/Gluconeogenesis gluconeogenesis], but not from fat.
* Eccentric exercise, such as [[Downhill Running]], can cause [[Delayed Onset Muscle Soreness| DOMS]] and impair glycogen replenishment<ref name="O'Reilly-1987"/>.
* Glycogen stores may not be replenished between daily hard runs, such as 10 miles at 80% of [[VO2max|V̇O<sub>2</sub>max]]<ref name="Costill-1971"/>.
At low exercise intensity the majority of the energy comes from free fatty acids in the blood, with a little bit of blood glucose and a little bit of muscle triglyceride. As the exercise intensity increases the contribution of free fatty acids drops. The contribution of blood glucose increases with exercise intensity, but not as dramatically as the contribution of muscle glycogen. At higher intensity muscle glycogen is the major energy source and is critical for performance.
[[File:Substrate usage over time.jpg|none|thumb|400px|Changes in substrate usage<ref name="romijn"/> over 120 min period at 65% [[VO2max|V̇O<sub>2</sub>max]].]]
At 65% [[VO2max|V̇O<sub>2</sub>max]], the usage of different substrates changes over time. The reduced usage of muscle glycogen may be due to a reduction in the availability of the glycogen. Over the two hour period shown, the fat:carbohydrate ratio changes from around 55:45 to 65:35. This change would reduce power output (running speed) at the fixed percentage of [[VO2max|V̇O<sub>2</sub>max]] (see ‘Glycogen Depletion and Breathing’ [[Breathing]]’ below).
=Glycogen Depletion=
The chart<ref name="selective"/> below shows that muscles do not become glycogen depleted at the same time. At all intensities shown, slow twitch fibers become depleted before fast twitch. The depletion within a fiber [[Fiber]] type is also not equivalent, with some fibers becoming depleted while others are fully loaded. This pattern implies a pattern of [[Muscle Recruitment]], where a subset of muscle fibers are recruited until they become exhausted, at which point other fibers are then used. As the slow twitch fibers become exhausted, fast twitch fibers are used in turn.
[[File:Glycogen depletion ST FT.jpg|none|thumb|800px|Glycogen depletion in human muscle fibers. The bars are colored with black indicating high glycogen content through to white indicating glycogen depletion. Three different intensities are shown; high (84% [[VO2max|V̇O<sub>2</sub>max]]) medium (64 %[[VO2max|V̇O<sub>2</sub>max]]) and low (31 %[[VO2max|V̇O<sub>2</sub>max]]) for each of Slow Twitch and Fast Twitch muscle fibers.]]
=Glycogen Depletion and Breathing Rate=
It requires more oxygen to produce energy from fat than carbohydrate<ref name="vent"/>. This may be why higher intensity exercise harder shifts to burning more carbohydrate. When our muscles become depleted of glycogen, muscles are forced to burn more fat. At any given exercise intensity we will use more oxygen when we are glycogen depleted. This means our heart rate [[Heart Rate]] will be higher and out breathing [[Breathing]] will be deeper and faster. It also means our perceived exertion is much higher for a given pace when glycogen depleted. This effect is most noticeable at the end of a long run or a marathon race, and it becomes much harder to stay on target pace. In fact, it can become up to 20% harder and this can be the difference between relaxed easy breathing [[Breathing]] and panting for breath. This [[Heart Rate Drift| increased demand for oxygen]] can often be seen in the [[Running Efficiency Calculator|calculated running efficiency]].
[[File:Ventilatory response and glycogen depletion.jpg|none|thumb|400px|This graph <ref name="vent"/> shows the relationship between a cyclist's power output and their breathing rate in normal and glycogen depleted states.]]
Carbohydrate
! colspan="2"|
[[Protein]]
! Fat
|-